. >> reporter: hitting the fast food drive-in for science. >> can i get five soft tacos. >> can i get a number four? >> reporter: over and over and over again. it's been dave giocolo's life the past three months or so. as has been eating the food. a precisely measured 1,000 calories a day -- an extra meal daily -- fast food only. >> how many calories? >> 770. >> reporter: why? because this man is paying him to. dr. samuel klein is a researcher at washington university medical school in st. louis, trying to understand why weight gain leads to diabetes and hypertension. at some point, research on rats alone just isn't enough. >> somebody has to eat this food this way to -- >> ultimately, it has to come to people. >> reporter: and a radio ad -- >> attention overweight volunteers -- >> reporter: -- that offered a cash incentive, up to $3,500. >> once i got into work, i called them right away. >> reporter: so did nurse dawn freeman. >> it probably took a month to get approved. >> reporter: and so they ate. every day -- from one of these five restaurants. until their weight went up a target